Moistureproofing paper



Patented Dec; 25, 1945 MOISTUREPROOFING PAPER Richard W. Quarles,Pittsburgh, Pa., and Arthur I K. Doolittle, South Charleston, W. Va,assignors to Carbide and Carbon Chemicals Corporation, a corporation ofNew York No Drawing. Application June 20, 1941, Serial No. 398,914

(or. 117-103) I 8Claims.

The invention relates to the moistureproofing of paper by theapplication of coating compositions, and it is particularly concernedwith a composite film coatin for paper, of high moisture resistance,which includes both vinyl resins and waxes.

The coating of papers of different kinds with vinyl resin compositionshas been heretofore proposed as a means for improving the imperviousnessof the paper to water, grease and other materials, as a method forenabling the paper to be joined to itself or other surfaces by heatsealing, and as a means for increasing the flexibility and resistance tocracking of the paper when subjected to the creasing encountered inwrapping and packaging operations. To increase the moisture resistanceof such coatings, waxes have been compounded with the vinyl resin. Thishas usually been done by dissolving the wax and the resin in a mutualsolvent to produce homogeneous lacquer-like compositions. Diiiiculty hasbeen encountered, however,- in obtaining adhesion of sufilcient strengthand permanency of'the films deposited from such solutions, particularlyto papers which have a hard and substantially nonporous surface, such asglassine. This difflculty as those of the glassine type, constitute apreferred class of paper for treatment.

Surprisingly enough, we have discovered by test that the above objectscan be attained by applying the wax and the vinyl resin as anon-homogeneous deposit on the paper surface. rather than from ahomogeneous solution containing increases if more than about 2% of waxis sought to be incorporated inthe resin. coating because the lack ofadequate compatibility of the resin with these amounts of wax usuallyresults in blushing of the film with accompanying brittleness and lossof film strength. To incorporate even such small amountsof wax in filmcast from a solution requires special selection and control of thesolvent compositions, the inclusion of predominating proportions ofaromatic hydrocarbons and esters which have good solvent powersfor'waxes, rather than the ketone compounds usually preferred assolvents for the vinyl resins used as coating bases.

It is an object of this invention to provide improved moistureproofcoatings for paper which include both vinyl resins and waxes. andto'provide such coatings in a manner which enables the essentialadvantages of each component to be utilized without interference withthe intended and most satisfactory function of the other. A fm-therobject is to provide an improved process for applying compositemoistureproofing films of vinyl resins and waxes on one or both surfacesof paper to produce a more strongly adherent and permanent coating thanhas heretofore been possible by ordinary means. Hard finished papershaving a substantially non-porous surface, such the wax and the resin,and thereafter briefly exposing this composite coating to hightemperatures. Twog'eneral forms of procedure are contemplated'ioraccomplishing this: one in which the wax is applied directly to thepaper surface prior to the vinyl resin coating and another in which thewax portion of the composite coating is applied in the form of adispersion of the solid wax in the vinyl resin coating solution. A veryquick bake of the composite coating at high temperatures is thenemployed in either instance to obtain the required adhesion andcontinuity of the coating. In the case of papers having a smooth hardsurface like that of glassine, the wax film is preferably applied beforethe vinyl resin coating. This may be done by melting the wax and formingit into a continuous thin film directly on the paper surface. Over thiscoating of wax a film of vinyl resin is then applied from a solution ofthe resin in solvents, which have little or no softening and solventaction on the wax, for instance, ketones, and the composite coating ofwax and vinyl resin is immediately subjected to an intense bakingoperation for a period of a few seconds and at a temperature in theneighborhood of 400 F'. The very rapid baking, and rthefact that thesolvents employed to disperse the vinyl resin have very little solventaction for wax, cooperate to prevent the wax from dissolving ordiffusing into the resin film and causing it to blush. A clear vinylresin film thus results, which has excellent adhesion tothe waxed papersurface, and which. covers and protects the wax mm to insuresubstantially permanent moistureproofness of the coated material.

In another modification, particularly adapted for coating papers of moreporous surface than glassine, the wax may be applied simultaneously withthe vinyl resin coating, by suspending or dispersing the wax in finesolid form in the solution containing the resin. For example, a solutionof the vinyl resin in suitable ketone solvents is prepared, and heatedto a temperature close to the melting point of the wax to beincorporated. Molten wax is then poured into the hot solution withstirring. As the solution slowly cools, the wax separates out in a veryfinely dispersed state, forming a fluid lacquer or composifrom about 5to 120 seconds inversely with the ticn which can be readily applied byspraying, roller coating or other means. This composition is thenapplied to the paper surface, and sub- Jected to an intense bakingoperation as mentloned above, producing an adherent, continuousprotective film having the desirable properties of both a vinyl resinand a wax coating. In this embodiment, it is likely that thewax forms aseparate coating underlying the vinyl resin film. In any event, thevinyl resin film is continuous and the dispersed particles of wax, if a.separate wax layer is not formed, do not aflect the strength of thevinyl resin film, and the maximum adherence and imperviousness in thecomposite coating is provided. This is to be distinguished from theapplication of the wax and vinyl resin from solution in a commonsolvent, wherein the vinyl resin does not form a continuous film and theresulting coating is brittle and weak.

When papers of medium or extreme porosity are to be coated, the aboveprocedure sometimes causes the wax, if large amounts are present, topenetrate the paper, resulting in an unsightly spot. This may beprevented by initially coating the pap r with a thin film of vinyl resincontaining no wax and then applying the vinyl resin solution containingthe dispersed wax as previously described.

The vinyl resins preferred for use in the invention are those whichresult from the conjoint polymerization of a vinyl halide and a vinylester of an aliphatic acid, and which contain from about 70% to about95% vinyl halide. Particularly desirable are the conjoint polymers ofvinyl chloride and vinyl acetate which contain about 80% to 95%vinylchloride, and which possess an average molecular weight of fromabout 9,000

temperature employed. The preferred operating range of temperatures isfrom 375' to 400 F.

At 400 F. a baking period of no longer than 20 seconds is usuallysufiicient to obtain suitable adhesion and brilliance of the film, butin any event the baking time should be correlated with the temperatureso as to avoid thermal deterioration of the resin. It is also importantthat resin solvents be selected which will not dissolve the wax to anyappreciable extent, and the commonly known ketone lacquer solvents, suchas methyl ethyl ketone, methyl isobutyl ketone, diisopropyl ketone, andisophorone, not only serve this purpose, but are recognized as excellentthixmer components for vinyl resin lacquers. The resin coatingcompositions may further incorporate, if desired, plasticizers,pigments,

to about 12,000, as determined according. ,to"

Staudingers method from the specific viscosit of dilute solutions. Lessdesirably, other vinyl resins may be employed, such as polyvinyl organicand inorganic esters or polyvinyl acetals.

Suitable waxes for use in the invention include the conventional waxeswhich have been used heretofore in moistureproofing, such as parafilnwax, ceresin wax, Japan wax, spermaceti wax, or the wax-like derivativesof chlorinated naphthalene. Particularly desirable are those waxes whichare highly incompatible with the vinyl resin, such as highly refinedparafiin wax melting at 165 to 170 F. The amount of wax used, in thesecond one of the above process embodiments can be varied according tothe degree of imperviousness to moisture desired. Even when the wax isincorporated in amounts of 10% or higher by weight of the vinyl resin,the coating has an excellent appearance and adhesion and the heatsealing characteristics of the coated paper are not impaired. Usuallyabout 2% to 10% of wax is sufficient to provide the required moistureresistance. Where the wax film is applied prior to the resin coating, itshould be ver thin, and preferably of a thickness not greater than about0.00015 inch, so that the finished coating may have an overall thicknessof not greater than about 0.0003 inch. Within this range, the resistanceof the coated paper to moisture will depend primarily on the actualthickness of the wax film and secondarily on that of the vinyl resinfilm.

In either modification of the process, an intense baking operation isessential to insure firm adhesion of the composite, non-homogeneouscoating of wax and vinyl resin. The baking time should preferabl be onlya few seconds, varyin fillers, and agents to stabilize the resin againstheat or light.

The following examples will serve to illustrate the practice of thisinvention:

Example 1.-A sheet of heavy calendered Manila paper was lightly coatedover one half its surface with a thick suspension of ceresin wax intoluene. After evaporation of the toluene, the entire paper was thencoated with a thin film of a conjoint polymer of vinyl chloride withvinyl acetate, which was deposited by spraying the paper with a dilutesolution of the resin in a mixture of aliphatic ketones. The vinyl resincoating was dried and the coated paper then subjected to a flash bakingoperation at 400 F. It was found that the vinyl resin fllm adheredtenaciously both to the wax-coated paper and to the uncoated paper andthe coated product was of excellent appearance and high gloss. However,the wax coated portion of the paper had much superior resistance tomoisture. This was shown by tests conducted at 35 C. on discs of thecoated paper having saturated water vapor on one side and an absolutelydry atmosphere on the other. The portion of the coated paper which hadnot received the wax treatment transmitted moisture at the rate 0! 6.6milligrams per hour per square centimeter, whereas the waxed portiontransmitted moisture at the much reduced rate of 0.53 milligram per hourper square centimeter.

Example 2.-Glassine paper was coated with a thin film of a high meltingparaflln wax by applying the wax from a dispersion in toluene andbriefly heating the coated paper to drive oil the a solvent and to flowout the wax. A very thin film of a conjoint polymer of vinyl chloridewith vinyl acetate was then applied by spraying over thewax coated papera dilute solution of the resin in volatile ketones; these solvents beingselected because they do not dissolve the wax. The total thickness ofthe resin film and the wax film was about 0.0003 inch. The coated paperwas then intensely baked for a few seconds at 400" F. resulting in afirmly adherent coating of high gloss. It transmitted moisture at thevery low rate of 0.013 milligram per hour per square centimeter whensubjected to the test previously described. Because of its highresistance to moisture and because of its ability to be joined by heatscaling, this coated paper makes an excellent wrapper or container forfoodstufls.

Numerous modifications in the invention as described will be evident tothose skilled in the art, and are intended to be included within thescope of the appended claims.

This application contains subject matter in common with that of ArthurK. Doolittle, Serial No. 349,235, filed August 1, 1940.

We claim:

1. A process of moistureproofing paper which comprises forming on thesurface of the paper a non-homogeneous deposit of a resinous vinylpolymer formed predominantly from vinyl chloride, and a wax incompatibletherewith, the vinyl polymer being applied from solution in an organicsolvent which is a non-solvent for the wax, and the amount of wax beingvery small relative to the weight of the paper and being between 2% andby weight of the vinyl polymer, and effecting a strong adhesion of thecomposite coatin: while maintaining its non-homogeneous nature by bakingthe coated paper at temperatures of about 375 to 400 F. for a period notexceeding about 120 seconds, whereby a clear and non-tacky compositefilm of wax and vinyl polymer is formed on thepaper.

2. A process of molstureproofing paper which comprises forming on thesurface of the paper a non-homogeneous deposit of a vinyl resin,substantially identical with that resulting from the conjointpolymerization of vinyl chloride with vinyl acetate and containing fromabout 80% to 95% combined vinyl chloride, and a wax incompatibletherewith, the vinyl resin being applied from solution in an organicsolvent which is a non-solvent for the wax and the amount of wax beingvery small relative to the weight of the paper and being between 2% and10% by weight of the vinyl resin, and efiecting a strong adhesion of thecomposite coating while maintaining its non-homogeneous nature by bakingthe coated paper at temperatures of about 375 to 400* F. for a periodnot exceeding about seconds, whereby a clear and non-tacky compositefilm of wax and vinyl resin is formed on the paper.

3. A process of moistureproofing paper which comprises forming on thepaper surface a film of wax having a thickness not greater than about0.00015 inch, covering the wax film with a film of a resinous vinylpolymer formed predominantly from vinyl chloride, the vinyl polymerbeing incompatible with the wax and being applied from solution in aliquid which i a non-solvent for the wax to form a composite coating notgreater than about 0.0003 inch in total thickness, baking the compositecoating at a temperature of about 375 to 400 F. for a period notexceeding about 120 seconds, whereby a. strongly adherent, clear,non-tacky and non-homogeneous composite film of the wax and vinylpolymer is formed on the with vinyl acetate and containing from about tocombined vinyl chloride, and subjecting the composite coating to a briefbaking operation at a temperature of about 375 to 400 F. for a periodnot exceeding about 20 seconds, whereby a strongly adherent, clear,non-tacky and nonhomogeneous composite film of wax and vinyl resin isformed on the paper.

5. A process of moistureproofing paper which comprises uniformlydispersing a wax in finely divided solid form in a solution of aresinous vinyl polymer formed predominantly from vinyl chloride in asolvent which does not dissolve the wax, applying a thin film of themixture so formed on the surface of the paper, subjecting the coatedpaper to a brief baking operation at a temperature of about 375 to 400F. for a period not exceeding about seconds, and forming an adherent,clear, non-tacky and non-homogeneous composite film of wax and saidvinyl resin on the surface of the paper.

6. A process of moistureproofing paper which comprises uniformlydispersing a wax in finely divided solid form in a solution of aconjoint polymer of vinyl chloride with vinyl acetate in a solvent whichdoes not dissolve the wax, applying a thin film of the mixture so formedon the surface of the paper, subjecting the coated paper to a briefbaking operation at a temperature of about 375 to 400 F. for a periodnot exceeding about 120 seconds, and forming an adherent, clear,nontacky and non-homogeneous composite film of wax and said vinyl resinon the surface of the '7. A process of moistureproofing paper whichcomprises uniformly dispersing a wax in finely divided solid form in asolution of a vinyl resin in a ketone solvent, said resin beingsubstantially identical with the product resulting from the conjointpolymerization of vinyl chloride and vinyl acetate, applying a thin filmof the mixture so formed on the paper surface, subjecting the coatedpaper to a brief baking operation at a temperature of about 375 to 400F. for a period not exceeding about 20 seconds, and forming an adherent,clear, non-tacky and non-homogeneous composite film of wax and saidvinyl resin on the paper surface.

8. A process of moistureprooflng a porous paper which comprises forminga thin film of a vinyl resin on the porous surface thereof, applying tothe resin coated surface a uniform dispersion of a wax in finely dividedsolid form in a solution of a vinyl resin in a solvent which does notdissolve the wax, subjecting the coated paper to a brief bakingoperation at a temperature of about 375 to 400 F. for a period notexceeding about 20 seconds, and forming an adherent, clear non-tacky andnon-homogeneous composite coating of wax and vinyl resin on the papersurface, said vinyl resin being substantially identical with thatresulting from the conjoint polymerization of vinyl chloride with vinylacetate.

RICHARD W. QUARLES.

ARTHUR K. DOOLI'I'ILE.

